Dental compositions comprising filler and resin matrix are widely used as dental materials for the restoration of portion missing from a tooth or for the production of dental prostheses including artificial teeth. For cured dental compositions, patients require good mechanical strength, hardness and adequate color tone comparable with their natural teeth as well as abrasion resistance in the oral cavity during mastication. In addition, dentists and dental technicians require dental compositions when they are in the form of paste, i.e. dental compositions before they are cured, to keep them being stable, easy to shape and easy to fill in a metal mold so that they can treat the paste easily. Dental technicians also desire cured dental compositions that can give grinding and polishing feelings similar to those of natural teeth.
Dental compositions are set from paste to give cured product in and outside the oral cavity. For example, a dental composition in the form of paste may be filled in an odontic defect and cured there in the oral cavity. Outside the oral cavity, a dental composition in the form of paste is shaped to a dental prosthesis and then cured. The important thing upon curing the paste into the cured product is volumetric shrinkage associated with the polymerization curing. Particularly, high inner stress and cracks may occur at the interface between the tooth and the dental composition or on the inside of the prostheses, resulting in low adhesion and poor mechanical strength of the filled materials or the prostheses.
Conventional dental materials had comprised quartz particles as filler of which particle size was relatively large, as large as about 150 μm. Dental materials containing such large size filler possessed practically acceptable mechanical strength. The degree of shrinkage of the material upon polymerization of the material in the form of paste was also practically acceptable. However, the cured materials could provide only unsatisfying grinding and polishing feelings to the dental technicians and insufficient abrasion resistance to the patients.
A dental material containing ultrafine silica particles as filler having a particle size from 0.01 to 0.05 μm was developed. The dental material was excellent in grinding feeling and can easily be processed to provide a smooth surface. However, the amount of the ultrafine particle filler in the dental material is limited due to high viscosity of the material containing a high amount of the filler when it is in the form of paste. Therefore, a cured product obtained from the paste of the dental material had unfavorable mechanical strength and abrasion resistance. In addition, degree of shrinkage of the material upon polymerization was also unfavorable. A hybrid-type dental material which had advantages of both aforementioned dental materials had been developed (JP-A 63-88110).
Various polymerizable monomers and polymerization initiators for resin matrix of dental materials also have been proposed to give improved dental materials that satisfy the aforementioned needs of dental technicians and patients. In addition to the improvement of resin matrix, polymerizable agents were also proposed and new dental materials were developed. Fillers of submicron average particle size were also developed and used for dental materials.